# Calculate the resistance

diredragon

## Homework Statement

In the picture below, is a circuit of constant direct current. The receiver is made of a changable resistor with resistance R. If the reciever delivers maximum electrical power its resistance is
a)9 b)6 c)3 d)1.5 e)0.5

## Homework Equations

Im having trouble finding the equation for this

## The Attempt at a Solution

I cant find the theory of this curcuit, the equations that fit so can you guide me a little in any way that could help?

#### Attachments

• image.jpg
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Homework Helper
Gold Member
Look up 'maximum power transfer theorem'. Are you familiar with the Thevenin's theorem?

diredragon
Look up 'maximum power transfer theorem'. Are you familiar with the Thevenin's theorem?
I am not but i will check it out now then try to apply something to the problem and get back to you :)

Homework Helper
Gold Member
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/thevenin.html
I read about it from here but am having hard time understanding how ti apply it to my problem.
For maximum power transfer, the load resistance should be equal to the Thevenin resistance of the circuit viewed from the load terminals. Here, your load resistance is the variable resistor. Can you proceed with this?

diredragon
For maximum power transfer, the load resistance should be equal to the Thevenin resistance of the circuit viewed from the load terminals. Here, your load resistance is the variable resistor. Can you proceed with this?
Is the equation for thevenin resistance the one in the picture uploaded? If so how to get it if not all three resistors are known?

#### Attachments

• image.jpg
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Homework Helper
Gold Member
If so how to get it if not all three resistors are known?
For computing Thevenin resistance seen from the load terminals, you should remove the load resistance (variable resistance) and compute the resistance of the circuit seen from the load terminals.